Color filters, which are important parts of liquid crystal display panels, have various requirements. When the color filter is employed as an STN or FLC liquid crystal display panel, heat resistance and chemical resistance are particularly important properties in forming a transparent conductive layer on the color filter and in subsequently processing the layer to form transparent electrodes.
That is, the above-described liquid crystal display panel is usually produced by forming a multicolor image layer on a transparent substrate made of glass, the multicolor image layer containing red, green and blue picture elements, and, in some cases, further having a black-colored or black shaded matrix; then, forming a protective layer thereon; and forming a transparent conductive layer thereon usually by sputtering of ITO. In the case of an STN type or a ferroelectric liquid crystal type panel, the transparent conductive layer is further processed to form a transparent electrode through photolithography to obtain a color filter equipped with an electrode. Then, an oriented film is formed thereon. That substrate and another transparent substrate having a transparent electrode and an oriented film are sealed, keeping a certain gap by using a spacer. Then, liquid crystals are poured between the two substrates, thereby preparing the panel.
In this series of steps for forming the color filter for the liquid crystal display panel, the protective layer on the multicolor image layer is required (1) to give it flatness, (2) to possess enough hardness to prevent the spacer from sinking into the layers in order to maintain the cell gap into which the liquid crystals are poured, (3) to show resistance to various chemicals used in the photolithographic process for producing the transparent conductive layer (solvents for a photoresist, acidic etching solutions, basic resist release solutions, solvents for an oriented film coating and the like), (4) to have transparency in the visible light region without cloudiness and turbidness, and (5) to show excellent adhesion not only to the multicolor image layer, but also to the substrate, because the protective layer on which no multicolor image is formed may happen to be directly formed. For this purpose, the use of thermosetting resins has been proposed, examples of which include epoxy compounds and polycarboxylic acids or their anhydrides described in JP-A-60-216307 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), nylon resins described in JP-A-60-244932, and melamine compounds and epoxy resins described in JP-A-63-131103. These thermosetting resins have certainly made it almost possible to satisfy the above-mentioned requirements (1) to (4).
However, there has been a demand to remove the protective layer from the transparent substrate in order to ensure sealing in the non-image portions thereof, and a desire that the protective layer on scribe lines must be removed as described in JP-A-57-42009, JP-A-1-130103, and JP-A-1-134306. It is difficult, however, that to conduct these treatments with high precision by using such thermosetting resins as described above.
Since such a thermosetting resin generally comprises a curing agent and an epoxy resin in combination, the mixing of these two reactive ingredients allows the reaction to proceed with time, resulting in an increase in the viscosity of the coating. Accordingly, this makes it difficult to keep the thickness of films uniform, and the pot life of the resin is short,resulting in an inability to obtain the precise film thickness.
So-called photosensitive resin types, in which the unexposed portions thereof can be developed, have come to be viewed as an effective means to solve these problems.
Examples of known photosensitive resins formed on color filter layers by techniques such as coating and employed as protective layers through photo-curing include ultraviolet curing type resins described in JP-A-57-42009 and JP-A-60-244932, vinylcarbonyl group-containing polymers described in JP-A-59-7317, photosensitive resins comprising PVA and photosensitive agents described in JP-A-59-184325, rubber-based resins described in JP-A-60-42704, and photosensitive resins in which pigments are removed from photosensitive resins which have the pigments dispersed therein and is used as coloring layers, as described in JP-A-2-191901.
Further, the most favorable protective layers are those which can be cured by light, whose unexposed portions can be developed by aqueous alkali solutions which are advantageous in hygienic safety and pollution measures, and which are thereafter cured by heating, to give higher alkali resistance. It is known that the use of compositions described in JP-A-3-126950, JP-A-52-132091, JP-A-2-97502, and JP-A-5-265208 makes such layers possible.
However, these compositions for forming photosensitive protective layers are still insufficient for developing the resulting sensitive coated film, in alkali resistance of the coated films, and in forming image after curing treatment subsequent to developing, in solvent resistance, in transparency, and, particularly, in adhesion to the substrates.